This invention relates in general to electronic motor controllers, and in particular to a method and means for applying more or less power to a polyphase AC induction motor as a function of a varying mechanical load on such motor whereby its power factor is optimized.
U.S. Pat. No. 4,052,648 to Nola discloses a control system for optimizing the power factor of a less than fully loaded AC induction motor. While the patentee directs his disclosure primarily to single-phase motor control, the application of his invention to three-phase motor control is briefly discussed wherein the patentee notes that three of his power factor control systems would be needed, i.e., a separate power factor control system for each phase. The patentee points out that such a three-phase application of his invention to a wye winding type motor would necessitate the provision by such motor of a fourth wire neutral. In the United States, most three-phase induction motors are of the wye winding type and a majority of these lack an external fourth wire neutral. Identifying the neutral point in the winding of such a motor to provide for a fourth wire neutral is extremely difficult, time-consuming, and impractical. The patentee further points out that delta winding type motors would have to be modified to include a semiconductor switch and a current sampling resistor in series with each of the three delta-winding legs. Such redundant systems and additional requirements for three-phase induction motor control undesirably complicate the achievement of improved power factor at reduced load conditions.